1. Field of Invention
This invention relates to optical detection of the contents of a pel in a gas panel.
2. Prior Art
Previous approaches have included optical isolation of a portion of the panel so that a pel was spacially segregated from another pel in order to separately interrogate them. This involves considerable complexity and expense in the optical system.
Alternately, electrical current sensing of the selected address is possible, however, it involves elaborate sense circuitry. An example is seen in U.S. Pat. No. 3,761,987.
Unlike cathode ray tube (CRT) displays wherein each pel is intensified at a unique time, gas panels frequently use a sustain system that energizes, i.e. "fires", all of the pels of the display simultaneously. Thus, unlike the CRT case, the pels of such gas panels are not differentiable with respect to time.
A solution to the problem is shown in U.S. Pat. No. 3,976,992. In that patent, the sustain signal can be "disturbed" with respect to one pel at a time by changing the shape of the normal sustain signal. This allows time-responsive "light pen" techniques, generally like those used with CRT displays, to identify a given pel. The particular method of "disturbing" the sustain signal so as to make it differentiable in that patent is to lengthen one half of the sustain signal applied to a given cell by adding a sliver of energy to the leading edge of a sustain pulse so that the resulting flash of light is out-of-cadence, that is comes earlier, than would be the case if it were undisturbed. However, the fact that the sustain signal is lengthened by use of a supplementary advance signal introduces requirements of precision, particularly in regard to exact juxtaposition of pulse portions from different gate circuits, which can be undesirable.
It is desirable to sense the information content of a pel or pels with minimum alteration of the normal operating conditions of a gas panel and to minimize the sensing equipment.